It is well-known that UV light or radiation, particularly from sunlight, can cause the degradation of polymers. Often, this results in the embrittlement or yellowing of polymers, which may be in the form of molded articles, polymer films, tapes, coatings, and the like. However, this degradation can be inhibited by the incorporation of ultra-violet light stabilizers and ultra-violet light absorbers in or on such articles.
The use of HALS and UV light absorbers ("UVA"), both individually and in combination, to stabilize polymeric materials is generally known in the art. In particular, UV light absorbers such as benzotriazoles and benzophenones were initially used to stabilize polymeric materials, and to prevent the degradation of such materials from exposure to UV light. Later, it was discovered that HALS, which scavenge free radicals formed in the polymeric material when exposed to UV light, were more effective than UV light absorbers, and thus, UV light absorbers are presently used in combination with at least one HALS in most conventional applications.
European Patent Application No. EP 0 704 560 discloses pigmented fiber of no more than 50 microns thick, in which the pigment is stabilized with a synergistic mixture of a HALS and a UV light absorber. The application discloses that the amount of both the HALS and the UV light absorber that is effective in stabilizing the pigment in the fiber is 0.05 to 5 percent by weight, but provides examples that show no significant improvement when the amount of HALS is increased over the amount of UV light absorber.
European Patent No. EP 0 453 396 discloses coating compositions containing from 0.01 to 5 percent by weight of a mixture of UV light absorbers containing at least one 2-hydroxyphenyl benzotriazole and at least one 2-hydroxyphenyltriazine or 2-hydroxybenzophenone, where the UV light absorbers are present in the molar ratio of 3:1 to 1:3. Optionally, the coating composition may contain a derivative of a polyalkylpiperidine HALS, such as 2,2,6,6-tetramethylpiperidine. Although a number of such polyalkylpiperidine derivatives are disclosed in EP 0 453 396, the amount of HALS useful in the claimed composition is not disclosed, and none of the examples specifically disclose a composition containing a HALS. All examples show some degradation after 1,000 to 2,000 hours of exposure to a Xenon Weatherometer, or after exposure to the sun in Florida for a period of about one year.
U.S. Pat. No. 4,619,956 discloses a method of stabilizing a polymer film, coating, or molded article against the action of light, moisture, and oxygen. The method comprises incorporating a HALS and a tris-aryl-2-triazine UV light absorber into the polymer. Preferably, the HALS is a 2,2,6,6-tetralkylpiperidine compound, salt, or metal complex, and the UV light absorber is a tris-aryl-2-triazine of formula ##STR1## where X, Y, and Z are each aromatic, carbocyclic groups, and at least one of the aromatic groups has a hydroxy group ortho to the point of attachment to the triazine ring. Each of R.sup.1 to R.sup.9 is hydrogen, hydroxy, alkyl, alkoxy, sulfonic, carboxy, halo, haloalkyl, or acylamino. Each of the UV light absorbers and HALS are used in an amount of from about 0.01 to about 5 percent by weight, but only formulations having equal amounts of UV light absorber and HALS are exemplifies. The compositions are effective in stabilizing the polymeric material, which does not begin to lose gloss or turn yellow until after about 1,000 to about 2,400 hours of exposure to UV light.
U.S. Pat. No. 4,331,586 to Hardy discloses oligomers for use as light stabilizers. While providing protection for polymeric materials such as polypropylene, polymeric films containing the disclosed oligomeric HALS became brittle after exposure to UV light for about 1,700 hours.
Therefore, a need remains for compositions and methods of use thereof that stabilize polymeric materials and provide protection from exposure to UV light for extended periods of time. The present invention provides such methods and compositions.